East of Napa Valley - metamorphic rock (slate)

Slate from the mountains of eastern Napa County, near Chiles Valley.

The mountains of eastern Napa County include large areas made up of metamorphic rocks of the Franciscan Complex (shown as KJfm on the geologic map), including slate.

Slate is a metamorphic rock. Metamorphic rocks are formed when sedimentary or igneous rocks are exposed to conditions of very high pressure or temperature, or are exposed to large amounts of very hot water. Slate is formed when fine-grained sedimentary rock (shale) is buried and exposed to high pressure deep beneath the Earth's surface. Slate, like many, but not all, metamorphic rocks is characterized by the way it breaks, along closely spaced parallel fractures.